A disk drive comprises a rotating disk and a head over the disk to magnetically write to and read data from the disk. The head may be connected to a distal end of an actuator arm that is rotated about a pivot to position the head radially over the disk. The disk may comprise a plurality of radially spaced, concentric tracks for recording data and servo information on the disk. The servo information may be read by the head to determine the position of the head over the disk and processed by a servo control system to position the head over a desired track.
During a write operation, a current is applied to a write element of the head (e.g., a write coil) to create a magnetic field which magnetizes the surface of the disk by orienting the direction of magnetic grains (e.g., horizontally in longitudinal magnetic recording, or vertically in perpendicular magnetic recording). The orientation of the grains exhibits hysteresis thereby generating their own magnetic field when the write magnetic field is removed. During a read operation, a read element of the head (e.g., a magnetoresistive element) transduces the magnetic field emanating from the disk surface into a read signal that is demodulated into an estimated data sequence.
The hysteresis of the magnetic grains is not permanent meaning that over time the grains will orientate into random directions (magnetic entropy) until the magnetic field is no longer sensed reliably (leading to data errors during reproduction). Magnetic entropy may also be precipitated by various factors, such as increasing ambient temperature. That is, at higher temperatures the uniform alignment of the grains will degrade faster. Another factor that precipitates magnetic entropy is a phenomenon referred to as adjacent track interference (ATI) wherein when writing data to a target track, the fringe field from the write element degrades the uniform alignment of the grains recorded in an adjacent track. The degrading effect of ATI on the adjacent tracks compounds over time with each write operation to the target track. Eventually, the magnetic field emanating from the disk surface will deteriorate to the point that the data is no longer recoverable.
To protect the integrity of data within an area of the disk against degradation over time, the data may be refreshed, in which the data is read from the disk and rewritten back to the disk. The refresh operation may be performed in the background, for example, after a certain number of writes are made within the area of the disk and/or other areas of the disk located near the area of the disk. The disk drive may perform refresh operations for many areas of the disk in the background to protect the data integrity of the disk.
However, when the disk drive is busy handling host commands, the user may experience an undesirable slow down of the command execution time while the disk drive initiates and executes refresh operations in the background. In a high duty cycle, heavy workload environment, the disk drive may need to throttle back user bandwidth in order to perform refresh operations. The refresh operations can sap the drive performance and at times shut down the drive.
Therefore, there is a need to protect the data integrity of a disk while reducing the negative impact of refresh operations on command execution time.